Design a multi-tenant notification/messaging system

Problem

Design a multi-tenant notification/messaging system. Multiple companies use the service, each needing several delivery modes and priority control.

Requirements

• Multiple channels: SMS, email, push, etc.
• Per-company and per-message-type priority management
• Tenant isolation across companies

Discussion points

• Pluggable channel providers behind a common interface
• Priority queues and per-tenant rate limiting
• Retries, fallbacks, and delivery tracking